Infection of the respiratory tract epithelium by aggressive viruses represents a significant biological threat to the healthy general public as well as to those with preexisting pulmonary disease. In individuals previously exposed to the virus, the recall response of T lymphocytes represents an important aspect of the mucosal host defense. It has been established that human bronchial epithelial (HBE) cells express a wide range of immune molecules necessary for providing primary and secondary (costimulatory) activation signals to memory CD4+ T cells. These include MHC class II molecules, the classic B7-1 and B7-2 ligands, and the more recently described B7 family ligands, B7-H1, B7-H2, B7-H3, and B7-H4. These latter, novel B7 ligands are believed to preferentially costimulate specific patterns of cytokine release from T cells, or to inhibit this activation, modifying the immune outcome. Despite these significant findings, nothing is known at present about the role of HBE cells in T cell recall responses to virus infection in the airways. The goal of this R21 application is to develop a fully-defined in vitro human syngeneic system in which the mechanisms of primary and secondary activation of CD4+ T cells by HBE cells can be investigated. We hypothesize that human bronchial epithelial (HBE) cells function as regulators of T cell recall responses in the airways. HBE cells will be grown to full mucociliary differentiation to provide a unique surrogate for the in vivo epithelium. These cells will be paired with T cells obtained from autologous lung tissue and directed toward a predominant MHC class II influenza virus epitope, HA306-318, associated with influenza A/Texas/I/77. Specific Aims include determination of the factors that (1) modulate presentation of the HA epitope and the expression of B7 ligands on HBE cells, (2) favor HBE-T cell costimulation within the epithelial structure, and (3) influence the profile of immune and inflammatory mediators released by both cell types. Fully differentiated HBE cells and a virus with well-defined immunologic features will be used to develop this cell culture-based model in which class II-restricted antigen presentation to activated T cells and their associated costimulation can be studied. The proposed studies involved with development of the model will themselves inform these processes as they relate to influenza A, as well as provide information that can be generalized to other biological agents, including more aggressive and emerging pathogens that target the respiratory epithelium.